Some similar compounds are eluted under NP and RP.
It surprise me that these compounds seem elute with same sequence under NP and RP ( from literatures) which suppose to be opposite.
Would anyone give me a explain? is that possible?

What compound class are we discussing? What were your conditions for each analysis?

I can see how it might be possible if you had a column that is commonly used in either mode, such as an amino or cyano column versus a silica column (rather than a C18 versus a silica column). I would think this could be quite possible if you're dealing with species that can carry a charge and you vary with your pH between both analyses, too.

So, what exactly are you up to?

Thanks!
The columns are silica and C18.
The compounds are fat acids ( long chain) with diol in the middle. The compound with -OH farest from -COOH side will come out first in both phase.
One explaination maybe that this compound is the least polar one, so it come out first in normal phase. At the same time, it has shortest pure hydrocarbon chain and this make it not easy to stick on C18 column and come out first in reverse phase. Is that right?

It looks like you have it.